The concepts of a hydrogen-cooled generator are very simple. As current flows in a conductor, heat is generated. A generator has a lot of conductors and a lot of current flowing through the conductors, generating a lot of heat. If that heat isn't "removed" then the windings will be damaged (insulation "blisters"; conductors grow and elongate causing clearance and balance issues; etc.). In addition, in a synchronous generator (alternator), there are high currents flowing in the rotor windings, which also generates heat which must be "removed".

Air can be used to cool a generator, by circulating it through the generator to absorb heat and then exhausting the air to another area outside the generator. A continuous flow of air from outside the generator, through the generator, to another area outside the generator will cool the generator and rotor. (The presumption is that the air entering the generator is cooler than the generator.)

Another way to cool the generator is to use hydrogen gas circulated through the generator and around the rotor to cool things. Hydrogen is seven to ten times better at transferring heat than air. That is, hydrogen is much, much better at absorbing heat and then at giving up that heat to another medium/area than air. This means that for the same size generator, if it's cooled with hydrogen versus air that more current can flow in the stator and rotor windings which means that more power can be produced. Or, thinking about it a little differently, the same about of power can be produced with a smaller generator cooled with hydrogen than one cooled with air, which is the typical reason for using hydrogen cooling--to reduce the physical size (and cost) of the generator.

The "problem" with using hydrogen as a cooling medium is that it's explosive when mixed with air and exposed to an ignition source. However, if the purity of the hydrogen is maintained at a very high level, meaning there is very little or no air in the generator casing to mix with the hydrogen, then even if there was a "spark" there won't be an explosion.

So, to use hydrogen to cool a generator it's necessary to prevent air from contaminating the high-purity hydrogen used to fill the generator casing after displacing the air. And that's important, displacing as much of the air inside the generator casing before filling it with high-purity hydrogen.

The hydrogen gas inside the generator is usually at a pressure of approximately 2 barg (30 psig), which means that air cannot leak "into" the generator casing where the hydrogen is. In effect, this is the primary method of preventing air from getting into the casing and contaminating the hydrogen.

The hydrogen is circulated by fans on the ends of the generator rotor, and as it's circulated around the generator it passes over coolers which have water circulating through them. The heat which is absorbed by the hydrogen gas as it passes through the generator and around the rotor is transferred to the water in the cooler. As the hydrogen exits the coolers, it's recirculated back to the generator and rotor, in a continuous cycle.

Another important consideration is to keep the hydrogen from leaking out of the generator, mixing with air and causing an explosion- or fire hazard where it might leak out of the generator.

The generator rotor rotates where it passes through the end-shields and that is the area that must be sealed to keep the hydrogen in and not allow it to leak out. A hydrogen-cooled generator uses "seals" to keep the hydrogen gas inside the generator.

The hydrogen seals are on the two shaft "ends" that pass through the generator end-shields. Oil is typically used as the sealing medium, and is sprayed on the shaft around the entire circumference of the shaft. The "seal oil" is at a higher pressure than the hydrogen inside the generator casing. Some of the seal oil flows out of the seal area along the shaft to the "air" side of the generator and some of the oil flows out of the seal area along the shaft into the "hydrogen" side of the generator.

The oil that's used as the seal oil is generally the same lubricating oil that's used for the bearings. That oil is normally in contact with air when it's in the lube oil tank and the bearing drains. So, air (in the form of small bubbles) can be entrained in the lube oil, and when sprayed on the generator shaft that air can be liberated from the oil that flows into the hydrogen side of the seal area. That air, if not "removed" somehow, can continue to collect inside the generator casing and reduce the purity of the hydrogen, and cause a safety concern.

So, because the hydrogen inside the generator casing is at a higher pressure than outside the generator casing, air can't leak into the generator. And, because oil, which will have entrained air in it, is used as the sealing medium, the air released from the "seal oil" that flows into the hydrogen area can reduce the hydrogen purity if not removed. So, the primary source of air to reduce hydrogen purity (contaminate the hydrogen gas inside the generator) is air liberated from the oil used to keep the hydrogen inside the generator from leaking out along the shaft.

So, there is a system to remove the air which is liberated from the seal oil that flows into the hydrogen side of the generator seal. That system is typically called a "scavenging" system. A small amount of gas is allowed to be vented from the seal oil enlargement tank, where the entrained air liberated from the seal oil is hopefully contained. That vent is normally piped to a safe area of the atmosphere away from any ignition source since it will have hydrogen gas in it as well as air.

Because a small amount of gas (air and hydrogen) is continually being vented to atmosphere through the scavenging system, the pressure inside the generator would decrease if nothing else were done. However, there is a pressure regulator from a source of high-purity hydrogen that maintains the pressure by flowing a small amount of high-purity hydrogen into the generator casing to maintain the pressure.

There is usually a hydrogen purity monitoring system which can be used to detect changes in purity during operation. Again, the presumption is that the hydrogen purity is high to begin with (after the generator casing is purged of air and filled ("charged") with hydrogen). But because of the air which can be liberated from the oil used as the sealing medium to keep the hydrogen from leaking out of the generator, it is necessary to have a means of monitoring the purity, usually at multiple points in the generator, to be sure that the purity is maintained to prevent a possible explosion or fire.

The scavenging flow rates must also be set and monitored to prevent excessive hydrogen consumption (to replace the hydrogen lost with the air that's being vented to atmosphere).

The seal oil that flows into the hydrogen side of the seal area is usually directed to an area called and "enlargement tank" and that's where it's hoped the entrained air is liberated and "contained", and vents from the enlargement tanks are directed through calibrated flow-meters to atmosphere (the scavenging system). The hydrogen purity monitors are usually capable of monitoring the purity of the gas in the top of the enlargement tanks that's vented through the scavenging system.

The hydrogen purity monitor can also be set to monitor the purity inside the casing (which should be higher than the enlargement tanks, theoretically).

Usually, when the hydrogen purity monitor indicates the purity is decreasing, then it increases the scavenging flow-rate to try to increase the purity, but venting more "contaminated" gas to atmosphere and replacing it with high-purity hydrogen.

If the hydrogen purity drops below a certain level (around 80% or so, depending on manufacturers' recommendations), then usually the generator and prime mover are stopped and then the generator is purged of hydrogen. (CO2 is usually used as the medium for purging air from the generator when filling with hydrogen, and also when purging the hydrogen from the generator.)

Presuming the hydrogen being used to maintain the pressure while scavenging is high purity, and because the casing pressure is usually two barg, the most likely way that purity can be decreased is from the air entrained in the seal oil. If the seal oil flow-rates (usually monitored with a single (unfortunately) flow meter) increase, then the air liberated from the seal oil will increase and that is the primary source of contamination and decreased purity. (This presumes that the purity monitors are working correctly.) So, it's very, very important to monitor seal oil flow-rates to detect an increase which would usually result in a decrease in purity. But, with only a single seal oil flow-rate meter for seals at two ends of the generator, it's difficult to detect which seal is degrading and consuming more oil which is liberating more air (but the decreased purity in a particular enlargement tank is the key ).

That should be enough of a description for a newbie or novice. If you have specific questions, we'll try to help. If you're working at a site with a hydrogen-cooled generator, you should be able to find the seal oil piping schematic (P&ID) which should be helpful in understanding the seal oil system. If the generator was manufactured by GE, it will usually have some drawings in the Instruction Manuals that are very helpful in understanding the systems in use on the generator at the site (though most are very similar, there can be subtle, but important, differences).

I read your message and simply superb. I am having small doubt that "What are the basic reasons for improvement of seal oil flow of the generator to maintain the same DP across the seal oil and the Hydrogen in side the Generator"

Oxford English Dictionary defines doubt as a feeling of distrust or disbelief; to question the truth of.

Nothing we say here is intended to mislead anyone. And it's perfectly acceptable in polite company to say, "I have a question", or, "I would like more clarification." But, to express doubt is to express distrust or disbelief.

I will answer the question of what would increase seal oil flow rates, and that is that the "seal" is actually a brass ring with an inner diameter slightly larger than the outer diameter of the alternator rotor. It is held tightly around the shaft with springs. Oil is admitted through ports and grooves in the seal rings and "sprayed" onto the shaft.

The brass rings and holding springs can wear or even fail over time. Dirty oil can also cause wear of the seal rings. The generator rotor shaft, as all shafts in journal bearing, moves as it is accelerated and even axially with the application of torque and electrical forces (though axial movement is generally very small, as is the radial movement). Unattended high vibrations of the generator shaft can also cause problems, as can induced voltages that are not properly grounded (through the shaft grounding brush). Also, the seal oil pressure regulators do require maintenance and adjustment and can be the source if increased seal oil flow rates.

We are happy to answer questions here at control.com. I imagine on occasion some questionable information is provided but never intentionally. The form of your question suggested it was not caused by any doubt, rather that you sought further information and clarification. Which we are happy to provide when asked, but not so happy to provide when requested by expressing doubt.

The problem with having a separate seal oil system is that at some point the seal oil and some of the bearing lube oil will eventually mix, and so I believe that's why the same medium (lube oil) is used for both bearing- and seal oil. Treating a portion of the lube oil is going to be costly and difficult, at best.

Decreasing H2 purity is primarily a function of seal oil flow rate, and it can be affected by moisture in the H2 and from leakage from the cooling water used in the H2 coolers. There are dryers and desiccants used to help remove moisture but a lot of machines don't have the dryers and only very small desiccant dryers which are rarely, if ever, properly maintained. Also, oil in the belly of the generator (from improperly charging methods and internal leaks) can also cause H2 contamination.

One has to remember: The casing is usually maintained at approximately 2 barg, or at some pressure above atmospheric. So, air can't leak "into" the generator; H2 can only leak out. So, the only real source of air is from the lube oil used as the seal oil medium. Presuming the purity monitors don't drift (and that's NOT a given with the old manual Wheatstone bridge used in many H2-cooled generator purity monitoring systems) about the only source of air is what's liberated from the seal oil.

Most H2 usage comes not from the scavenging replacement, but from leaks--leaks of H2 out of the generator casing that cause the pressure to decrease. Actually, this should help increase the H2 purity because of the higher replenishment rate due to the leakage, because, again, air cannot leak into the generator.

There are many sites around the world which have opted for on-site H2 generation. It just adds another system to be operated, monitored, and maintained. And in my experience, they were never operated or maintained properly and so most of them were soon shut down and bottled H2 was used for the generators.

The first question that should be always be asked (in lower-case letters, thank you very much!) is: When did this problem start?

If you're consuming large amounts of hydrogen and have high scavenging rates, it could be the seals and/or seal springs are failing or are excessively worn. I've heard that the float mechanism in the float trap can cause this, but I've no personal experience with that.

I've also seen brand new generators with new L.O. have foaming all during commissioning, and I don't know if it ever lessened or stopped. I think some formulations of turbine oil are more subject to foaming under some conditions than others.

We don't know if yours is a new unit, with new L.O., or if the L.O. was recently replaced with a new or different formulation from the same or a different manufacturer. We don't know if there was a recent maintenance outage when work was done on the generator end-shields or seals.

I also have a suspicion that sometimes the "foaming" comes from some higher pressure somehow downstream of the seal oil float trap that is making it's way back into the trap, but I'm not exactly sure how that could happen. I only say that because I witnessed bubbles rising through the sight glass in the float trap. And that's the only reason I could come up with. But, the bubbles stopped in a couple of days (new unit being commissioned). It could have just been gas trapped in the piping somewhere; it was never investigated completely.

But, it would be helpful to know when this problem first started and how long it's been persisting and anything you might have done to try to investigate and troubleshoot the foaming.

> The problem with having a separate seal oil system is that at some point the seal oil and some of the bearing lube> oil will eventually mix, and so I believe that's why the same medium (lube oil) is used for both bearing- and seal> oil. Treating a portion of the lube oil is going to be costly and difficult, at best.

---- snip ----

> There are many sites around the world which have opted for on-site H2 generation. It just adds another system> to be operated, monitored, and maintained. And in my experience, they were never operated or maintained> properly and so most of them were soon shut down and bottled H2 was used for the generators.

what are the possible causes of oil coming out from generator water detector?

Failing hydrogen seals; improperly installed hydrogen seals; failing or broken hydrogen seal springs; excessive Seal Oil pressure are the usual culprits. Dirt/debris has also been known to get into the seals and cause problems, usually the brass rings get scored and sometimes the shaft, also. This is an extreme case, but has happened.

We have intermittent oil leakage excursions that collects in the water drains. sometimes it is one end and other times it tis the other. This is a Westinghouse machine with air and H2 seal oil and air cooled. We have yet to correlate it to any transient parameter such as temperature or pressure. Are there seal failure modes that are transient...such a dirt causing temporary seal "hang up"?

>Failing hydrogen seals; improperly installed hydrogen>seals; failing or broken hydrogen seal springs; excessive>Seal Oil pressure are the usual culprits. Dirt/debris has>also been known to get into the seals and cause problems,>usually the brass rings get scored and sometimes the shaft,>also. This is an extreme case, but has happened.

it is really very helpful for me. sir i have one question, what is the function of liquid level detector (i am fresher in CCPP and working in the Maintence side, ours is ge 9fa machine)? is this any how related with float trap assembly).

A liquid level detector is for detecting any liquid in the generator casing.

There shouldn't be any liquid in the generator casing under normal operating conditions.

Sources of liquid could be leaking hydrogen coolers and excessive seal oil flows.

Please find the Generator Hydrogen System P&ID drawing. Also, if you have a unit packaged by GE you should have a section in the Instruction Manuals that describes, in very brief detail, how the system works. It's not much, but it's better than nothing.

Learn to use the available documentation at your site. We can fill in gaps and details here at control.com.

Very interesting information. Any chance you know (or can point me in the right direction) to learn about how much power the whole cooling system draws? That is, how much recirculating power is there?

Thanks.

> The concepts of a hydrogen-cooled generator are very simple. As current> flows in a conductor, heat is generated. A generator has a lot of conductors and a lot of current flowing through the>conductors, generating a lot of heat.

Hello my Name Is Maroun and i am planning to an invention of Hydrogeen cooling system for House kitchens, any tips on that? I am not that experienced in that Field but just have the Idea to develop. Thank you

Of late we have facing a problem of low Dissolved Oxygen (1.1 ppm) in Stator water system. Hydrogen consumption has increased as compared to other running units. I would like to know how to collect sample from Gas trap and what analysis to be carried out to confirm the hydrogen leakage?

However, the way most machines are built, meaning the way the internal piping for charging/purging is constructed, CO2 is chosen because of it's weight versus hydrogen and air.

Because CO2 is heavier than air it is "injected" into the bottom of the generator casing to push the hydrogen up and out the top of the generator casing.

When using air to purge the CO2 from generator casing, it is "injected" into the top and used to push the CO2 out the bottom of the generator casing.

You might also need to have different casing gas monitoring equipment for purging and charging as most monitors measure for the presence of H2 in air, or H2 in CO2, or similar. I've never seen one that measure anything in N2 or N2 in anything (for generator casings, that is).

You should study the internal piping arrangement drawings for the generator(s) at your site, and then analyze the situation to see if you could make this work. Let us know how you fare.

I attended a meeting some years back where Linde discussed using argon instead of CO2. More expensive, but vaporizes faster and therefore they claimed that both degas and regas cycles could be faster. Suggest you touch base with them if you are interested.

I read this article and found it very interesting. It will be great if I could know how much hydrogen (in Tons) is required for 270 MW GT cooling function and how frequently the hydrogen used in the GT need to be re-charged.

The answer to this kind of question depends on many factors. The size of the generator casing, including the gas-water heat exchangers, drain enlargement tanks, and associated piping. Also, the scavenging rate (the amount of gas vented from the seal drain enlargement tanks to remove entrained air) will affect the amount of hydrogen which is required to keep the pressure at or near rated.

And, scavenging rates usually increase with wear on seals or increasing shaft vibration. So scavenging rates are constant over operating periods; they will change as components wear, and are replaced.

If you have a particular generator "in mind" that will be in use at the plant, or is in use currently, you will need to find the manufacturer's or supplier's data sheets for this specific information.

Very nicely written explanation. Please answer two quick questions about the cost of keeping large generators cool during operation. Depending on the generator, where it was used, and how it was used, you might need to use hydrogen cooling. If a generator costs $100,000, what might be the estimated cost of its hydrogen cooling system in round figures? Regardless of how you cool a generator, there will be an energy cost. If a generator were to produce 100 kilowatt hours of electricity, what would be the expected range in kilowatt hours of electricity that might be needed to operate the generator's cooling system?

CSA…I am glad this thread got bumped. That was an excellent explanation with some very good information and presentation.

One of the questions as reading through replies was the use something to prevent the need to scavenge. The LSTG units do use a vacuum treated oil system. Here is my explanation written to dispel some false assumptions on how the lube oil is used with this system.http://www.slideshare.net/JosephFByrdJr/seal-oil-operation1

In my 3 decades, I have only passed by a few savaging units. However, if the vacuum seal oil system has a failure and untreated oil is used, the unit must be savage. Unfortunately the vacuum units are not constructed for a proper valving to do so. I wrote this paper for a customer to explain the concerns of operating for long period without vacuum treaded oilhttp://www.slideshare.net/JosephFByrdJr/seal-oil-operation1

I then wrote up this paper discussing the amount of H2 expected to increase purity.http://www.slideshare.net/JosephFByrdJr/scavagine-h2Since I have no experience with scavenging units, do you think the rates I predicted are valid or was my assumed absorption % too high.

Last point related to cost, the pumping and reduction of cooling losses related to every % the purity is less than 99% is considerable. I believe "environmental one" has a good write up in support of their scavenging controls

the information helped me a lot to understand this system. thank you so much. but i still have one question. i am working on site and its siemens make hydrogen cooled generator. enlargement tank vent is connected to seal oil hydrogen side return. why is it so?? because seal oil hydrogen side return is also connected to enlargement tank. i will provide more info if you need to clear my question.

I don't know if I understand the question completely. I have seen a setup with a Seal Enlargement tank on the TE and CE of the generator. Both of these Seal Enlargemnet tanks drain into a float trap via an internal weir system. When the float trap rises enough it dumps the oil into a larger tank. To prevent vapor lock on the float trap a reference line (vent) is installed back to the CE Seal enlargement tank. The larger tank has an atmospheric tank and also receives the bearing lube oil. In the event there is a high rate of oil through your seals and there isn't enough resonance time in the small Seal Enlargement tanks the larger Bearing/Seal Enlargement has a seal leg design allowing any remaining entrained hydrogen to vent off before returning to the main reservoir.

I am deeply impressed by your knowledge on generators and their cooling systems.

I am currently conducting a market study in this field and wanted to ask if you would be available to conduct an interview with me (remuneration would be a 75USD Amazon gift voucher for a 45min consultation).

When did this problem start? After a maintenance outage? After a purge/charge event?

What kind/size of generator? Does it have stator cooling?

Have you changed hydrogen suppliers? Do you produce your own hydrogen?

Have you tested the condensate/moisture to see if it might be coming from a leaking cooler? Or stator cooling system?

Does the hydrogen have a hydrogen dryer? If so, has it been used recently?

Really, there are only so many ways for moisture to get into a pressurized vessel. Air is used to purge CO2; CO2 is used to purge air; hydrogen displaces CO2; moisture is entrained in seal oil (usually bearing lube oil); water is used to cool hydrogen; water is used to cool stator bars (in very large generators. If moisture gets introduced in any of the methods, and if the generator gas temperature gets too low then it can condense--unfortunately on generator internals. Keeping hydrogen free of moisture is very important. Most hydrogen control panels I have worked on have had moisture indicators (actually, desiccant that changed color when wet) which can indicate the presence of excessive moisture (if quickly changing colors). These are only minimally effective at removing moisture, but for most normal circumstances are adequate.

Again, moisture in air/CO2/hydrogen, and moisture from leaking coolers/stator cooling systems, and moisture entrained in seal oil are about the only suspects.

In thinking about this, has there been a drop in the cooling water temperature recently? And possibly a reduction in load since the problem started?

Both could result in condensation of moisture that was previously "suspended" in vapor in the hydrogen because the gas temperature was above the dew-point but now is below the dew-point.

Also, does the generator have a hydrogen dryer (usually electrically powered) that could be used to remove entrained moisture? If so, be sure to observe the proper purging sequences before and after use.

After reading your post, I think you are the one who can help me to get the answer for my question. I want to know that is gasmeter used to detect the hydrogen leaking from the rotor? Do you know where the detectors are placed to monitor the hydrogen leaking from the rotor through the seal oil system?

There should be P&IDs provided with any GE-design heavy duty gas turbine generator which would show the presence--not the exact location, but the presence--of any hazardous gas detector installed on the unit (turbine or generator). Sometimes it's shown on the Fire Protection P&ID for the turbine-, accessory or load compartments, and sometimes it's shown on the Generator P&ID.

That's if there are hazardous gas detectors--which check for the presence of combustible gases.

It wasn't until a few years ago that GE began providing hazardous gas detectors to detect hydrogen leaks along the shaft (through the hydrogen seals) of a generator. Sometimes they are located in exhaust ducts of the compartments (Load or Generator Collector) because the air exiting the compartments is "funneled" through the exhaust ducts and that makes it easier to detect the presence of high levels of combustible gases--as long as the hazardous gas sensors are kept clean and oil-free (that includes oil from the hands of well-meaning technicians!). And, that's difficult to do when high volumes of air are continually passing over the sensors.

So, you need to check the hazardous gas sensors for your unit (if there are any), and determine which compartments they are located in, and then go to the compartments and find the sensors to determine exactly where they are located. At least for GE-design heavy duty gas turbines.

Hope this helps! When you write for help, please try to provide as much information as you can about the equipment in question. It will help us to get you as concise a reply as possible in the quickest amount of time. Be aware, though, that while there are some very knowledgeable people here replying to threads on control.com we haven't seen every piece of equipment or configuration, so sometimes if you don't get a response it's because someone who's familiar with the equipment you describe may not be reading your thread today, or this week, or this month. There is a good deal of experience with GE-design heavy duty gas turbines here on control.com, but not so much for other manufacturers' equipment. Many are similar, but some are very different.

Please can someone tell me, what is an alternative to a stableflow (tm) hydrogen controllers manufactured by proton? or alternatively who all are manufacturing a hydrogen controlling system for generator cooling?

I don't understand the question. If you're saying that hydrogen consumption exceeds the amount "specified" in the manual, then isn't that excessive?

If hydrogen consumption is increasing over time, isn't that an indication of some problem or issue which needs investigation to understand and resolve if necessary?

If hydrogen consumption has been below the level specified in the manual and is now increasing and has exceeded the level specified in the manual, isn't that excessive hydrogen consumption indicative of some problem or issue which needs investigation?

At what point will the cost of hydrogen increase to an unacceptable level?

Or, more correctly, if there is an increase in hydrogen consumption from past levels and it's not known where the hydrogen is leaking/accumulating, isn't that reason enough for investigation and resolution to prevent a fire or explosion from accumulated hydrogen gases?

Every site has to make this determination based on their tolerance for hydrogen leaks and the cost of hydrogen to replace that being lost. There are sites that replace hydrogen bottles/cylinders every two hours and seem to think that's acceptable--and they have no idea where the hydrogen is "going." There are sites that won't accept even the level specified in the manual--and can't calculate the amount flowing through their hydrogen analyzer/scavenging system to see if that's the cause of the consumption (in other words, there are sites that don't think they should be replacing any hydrogen at all, or, if they do, it should be one bottle/cylinder a month or less!--for these sites they just don't want to spend any money replacing hydrogen, it's simply an economic "consideration" for them).

To my way of thinking, losing hydrogen (hydrogen consumption) without knowing where it's going/leaking/accumulating is bad. It's a safety concern. If you've ever seen the results of a hydrogen fire you would understand. When trying to understand this kind of problem, one has to understand a lot about the current condition versus the condition when the consumption was lower--was the generator casing recently opened for maintenance/repair? Has someone changed the scavenging flow rate(s)? Has seal oil flow-rate increased? Has there been a problem with cooling? There are a lot of factors to consider, not to mention the rate of increase of hydrogen consumption. How fast is it increasing?

But, it's not possible to provide a number or percentage at which someone should be concerned--it's always a subjective value, meaning it's subject to the knowledge, experience and concerns of management.

I was searching for some information & I came across this wonderful & very informative thread.

We are operating GE Frame 7FA units with H2 cooler generators, where we are maintaining H2 pressure at 30-31 psi, with purity 98-100%.

Of late 1 of my units has high "Generator common gas temperature" 156 deg F, as compared to other units which have temperatures 115-120 deg F. H2 pressure & purity has been steady for this unit, with last purity improvement being carried out on 14th Jun2015.

I checked the cooling water circuit & found it to be pretty normal with pressure & temperatures normal across all units.

Are you sure the "Common Gas" temperature is inside the generator casing?

When did this problem start? After a maintenance outage? After a trip from load (high load)?

I don't have any F-class Generator Package Wiring Drawings to refer to, but if I recall correctly there are some RTDs used to monitor the air entering the collector housing and the air leaving the collector housing--and they are given some less-than-descriptive names and locations in the documentation.

If the stator temperatures in the affected generator haven't changed much, if at all, since this high "Common Gas" temperature indication started, and if the gas temperatures entering and leaving the coolers haven't changed much, if at all, then I would suspect the RTD(s) being referred to are not located inside the generator casing, but outside. Dirt and grime can collect on RTDs in the collector housing area very easily and cause problems with readings. Also, if people don't take care when working in the area the RTDs/leads/wiring can be damaged.

You need to find your Generator Package Wiring Drawing to find the locations of all of the RTDs in and on the generator, and then look at the specific RTDs which are indicating this high "Common Gas" temperature, and decide how to proceed.

IF the "Common Gas" temperature RTD(s) are indeed inside the generator casing, then I would suspect there is some issue with one or more of the RTDs used to derive this temperature, OR with the wiring between the generator JB and the Speedtronic. Again, if the stator temperatures and the other internal generator casing temperature measurements haven't changed at the same time as this high "Common Gas" temperature indication started, then it's either a sensor (RTD) problem, or a wiring problem, or a Speedtronic input problem (which would likely be accompanied by a related Diagnostic Alarm).

The only time I've seen this occur before (also on a multi-unit site) was when air in the cooling water system made its way into the hydrogen coolers and got trapped in the top of the coolers, reducing the efficiency of the coolers significantly. BUT, in this case, all of the other internal generator casing RTDs also indicated a loss of cooling (meaning the stator temp's increase, the inlet- and outlet cooler gas temp's also increased).

I have seen pictures of some winding insulation that came loose and migrated to different locations inside the generator casing causing problems with temperature measurement. But, without knowing a lot more about exactly where this Common Gas temperature measurement is being taken, and if other temperatures have also gone higher, there's not much more to recommend.

Far too many hydrogen-cooled generators get a "dose" of lube oil during initial purging/charging when inexperienced commissioning personnel (mechanical and controls, working "together") incorrectly set up the seal oil valving. This can lead to some delayed reactions as oil and sludge, which can't be removed from the generator make their way around the internal passages and collect in unusual places. (These kinds of incidents rarely, if ever, make it into the Start-up Report, so unless someone was around when the units were commissioned this isn't generally known.)

This also occurs during initial purging/charging activities by site personnel as they get accustomed to the operating procedures required for this activity. Unfortunately, when site personnel accidentally "dose" the generator with some oil unless the Gen. Casing Liquid Level alarms go off, usually the oil doesn't get drained out, or doesn't get drained out as well as it should have been....

It would be great if you would write back with the exact location of the RTDs used to measure the "Common Gas" temperature (from the Generator Package Wiring Drawing), and the exact names of the RTDs used in the determination in the Speedtronic. Also, if the Common Gas is indeed inside the generator casing, it would be helpful if you could tell us if the stator temps have also trended higher prior to this Common Gas high temp indication, as well as the cooler inlet- and outlet gas temp's.

But, this should help get you started in your understanding and search. There isn't too much that can go awry in the generator casing.

Look forward to hearing back from you with more information, and maybe that you've found the resolution.

My unit is a Siemens 1000 MW one and works slightly different.I'd like to know, If possible what's the correlation between cold gas temperature and H2 purity, i.e.How much cold gas temperature would decrease in C for an 1% H2 purity improvement?

Hydrogen is much better at transferring heat than air--that's why one of the biggest reasons it's used for generator cooling. However, in my personal opinion--because I've never trended cooling and H2 purity--the relationship between them is very small, especially considering the normal range of purity is technically supposed to be very high (above 97%). Perhaps if the purity was much lower--indicating more air or other gas (CO2 not well purged during charging) or air/contaminants from leaking gas-to-water heat exchangers) there would be a larger change in cold gas temperature for a smaller (1%) change in H2 purity( which would mean less air/gas/contaminants which is less effective at transferring heat).

Having said that, a 1000 MW (1 GW) machine is a big machine--with lots of gas volume. It's possible that a 1% change in purity would mean a pretty big change in air/hydrogen concentrations and that could represent a larger change in heat transfer than for a smaller machine--say, 250 MW, or 125 MW, or 80 MW, or 30 MW.

This question was asked recently--though I'm not sure if it was on control.com. And the respondent said basically the same thing: There's no direct correlation between H2 purity at cold gas temperature, and I believe that's mostly true because normal purity is supposed to be maintained in the high 90 percent range. Again, I could imagine if the purity was in the low 90% to high 80% range (which is not recommended) that with less hydrogen and more air/CO2/contaminants a 1% change in H2 purity might make a larger difference in cold gas temperature.

But, it's interesting you're not saying what H2 purity the 1000 MW machine is being operated, and if there are problems with generator temperatures prompting the question about increasing purity and decreasing cold gas temperature and hopefully decreasing generator temperatures. Is there a problem with generator temperatures? More to the point, is there a problem with achieving H2 purity in the high 90% range?

Could there be problems with H2 coolers? Cleanliness?

Explaining the issues--presuming there are issues--would probably yield answers which would be more helpful, again presuming there are issues. But, again, there is typically no direct relationship between a small change in purity and cold gas temperature.

About the best answer for your turbine is to trend purity and cold gas temperature--presuming cooling water temperature/flow-rate is relatively constant as well as load (real and reactive).

We use to operate our machines in the lowest region of the allowed band,that is 95% H2 purity for us. This summer has been so hot and we have experienced some trouble to keep cold gas temperature within the recommended 46C, so we've had to lower power to meet the Temperature limit. Later on, we decided to increase H2 purity to a 98% and it worked fairly good, so, full power could be recovered. Some still think that such a small detail alone can be responsible for the cold gas temperature decrease we've obtained, and I'd like to know if anybody has experienced a similar situation to confirm or discard this fact.

> About the best answer for your turbine is to trend purity> and cold gas temperature--presuming cooling water> temperature/flow-rate is relatively constant as well as load> (real and reactive).

This is what I've done in my unit and I've found a rough 0.8 C cold gas decrease for every 1% increase in H2 purity. I'd like to know if this figures could be right enough.

I think we're kind of "uncharted territory" for operations, as, again--it's generally regarded that there isn't a direct correlation between purity and cold gas temperature. But, as was said above, with a large machine and a lot of volume it seems likely there would be some relation between the two--and by trending the signals you have likely developed the relationship for your machine.

I would say it's not possible for anyone other than the machine designers to put a firm number to the relationship between purity and cold gas temperature, but all other variables seemingly remaining relatively constant it's reasonable to presume the decrease in cold gas temperature can be attributed to the increase in purity. That's probably about all that can be said without analyzing the data.

"Another important consideration is to keep the hydrogen from leaking out of the generator, mixing with air and causing an explosion- or fire hazard where it might leak out of the generator."

thanks for the interesting information and help on this interesting topic.

Regarding this short notice of hydrogen leaking out of the generator, I was wondering, if anyone here has ever experienced hydrogen leaking through the sockets of thermocouples which are measuring the hydrogen temperature and leaking out, resulting in (luckily currently very very low) amounts of hydrogen in the control cabinet.

Does anyone have any experiences or good ideas, how to fix the insulation on those sockets without releasing the hydrogen?

It's very helpful article for fresher like me. I'm first time involved in the basic design of Hydrogen cooling system, could you help me by providing basic guide line documents, design steps, calculations , or any presentation?

Can we use nitrogen instead carbon dioxide to purge the hydrogen? as both nitrogen & carbon dioxide are inert gas, but the density of CO2 is higher than the nitrogen. but nitrogen is still heavier than hydrogen.

People have tried it before with very mixed results. It depends on several factors, first, the patience of the people purging and charging the generator (which is usually not very good, and neither is that of management waiting for the purge and charge to be completed).

Second, the instrumentation available. Usually one of the meters is set to read CO2 in Air, or something similar, so I believe it's not really expecting any inert gas.

Third, I think it's particularly difficult to purge the casing of air with nitrogen before charging it with hydrogen.

In my personal opinion, purging and charging (regardless of the gases and instrumentation used/available) needs to be done slowly if for no other reason than to reduce the mixing of gases during the processes. Slow and steady wins this "race": the race to purge the casing of one gas as completely as possible and then fill it with another as quickly as possible with the highest purity possible. Leaving any air in the generator casing when charging, whether CO2 or nitrogen was used for purging, is not desirable.

A lot of people ask why the purity goes down during initial operation after a purge/charge, and it's likely because of two things. First, the scavenging rates need to be adjusted, and second, because it's likely that the purge and charge wasn't as effective as believed because of the rush to complete it. So, it usually takes a little more scavenging to get the purity up, and then it can be adjusted lower.

That's the part that most people don't understand about scavenging: It's b>NOT 'set-it-and-forget-it.' It needs to be adjusted over time to maintain purity, and ANY adjustment needs to be logged for tracking and trending purposes which can be very useful when trying to determine if hydrogen usage becomes excessive. And adjustment can be to decrease scavenging as well as to increase it. Remember: The adjustments are made to maintain purity. And any adjustment is probably going to take several hours to seen any effect; it's not immediate.

So, patience is the key.

But, to answer your question simply, it's your machine and you're free to try it. It's been done with mixed success, and if you're willing to take the chance and live with the consequences, then it would seem to be feasible. But, then, a lot of things look easy "on paper", don't they?

Honeywell has made a thermal conductivity analyzer for hydrogen cooled generators for decades. The thermal conductivity cell has not changed since the 1970's.

The electronics box is known as a Triple Range Analyzer because it provides concentration readings for running and purge cycles.Range 1: 0 to 100 % CO2 in AirRange 2: 0 to 100 % H2 in CO2Range 3: 0 to 100 % H2 in Air

The electronics that determine the concentration from the thermal conductivity on-line cell raw data definitely expect H2, CO2 or air. not nitrogen in place of CO2. That doesn't Nitrogen can't be used, but any thermal conductivity analyzer expecting CO2 will read wrong when used with nitrogen.

This is a great thread. Does anyone have any data on the effect of hydrogen purity of the Generator. What would be loss of efficiency of a Generator if the purity of H2 begins to decrease.

On another note. I have worked on many station in the middle east where they have on site Generation. What I found was that if you select the right Company, you can make your own hydrogen for years and years. But there are many more failed On-site Generation Companies. Make sure you select the right one. There are only 3 that come to mind.

> I have a PowerPoint presentation I can> send you but I do not know how to add> attachments to the site, Do you know?

Is it possible to get a copy of your presentation that you so graciously offered. Either a native format or pdf will be greatly appreciated and can be sent to mr2jzherring at aim dot com (I don't know if real emails get through). And thank you in advance.

Hi I'm interested in this post as I've made a domestic computer cooling chamber that cools to about -25c. The system uses recirculated air as a cooling medium and I wish to replace this with another gaseous medium that is more effective.

While hydrogen would be appealing due to it's 10x fold increased cooling efficiency I fear it's explosive nature would not suit it for domestic use.

I was wondering if anyone has information of other gases that would be more efficient than air but safer for the domestic environment.

The cooling gas in the generator should be dry, to protect the windings from moisture being circulated in the cooling gas.

Sources of moisture can include leaky hydrogen coolers and excess moisture entrained in the Seal Oil (along with entrained air--the air can be humid, especially if the turbine driving the generator is a steam turbine and the same oil is used for both bearing lubrication and Seal Oil.

Regardless, there should be a vapour extractor on the oil tank (L.O. or Seal Oil) to help remove any air/gases as well as any humidity.

Again, the idea is not to allow moisture to condense on the generator internals.

Poor purging/charging and contaminated hydrogen can also cause increased moisture in the generator casing. If the hydrogen is produced on site by a local hydrogen-making system they have been known to be problematic. Also, some areas of the world have been known to receive poor quality gas from their suppliers (who tests the gases they receive??? or who even looks at any quality documentation (which usually isn't supplied with many gas cylinders (with the exception of emissions calibration gases--which have also been found to be of poor quality and not within specification).

If a generator is equipped with a hydrogen dew point detector, it probably has a hydrogen drier as well, or at least a way to circulate hydrogen from the generator casing through some kind of drier (absorption beads; etc.). Hydrogen driers are used to remove moisture from the generator casing gas. (Remember to follow all purging and operation procedure before and after running the generator casing gas through any drier vessel and associated piping!)

we are operating 7FA units in saudi arabia with hydrogen cooled generators. our hydrogen dew point entering to dryer is -30 degc. is this dew point is OK? what should be the ideal dew point temperature of Hydrogen gas in the generator?

I'm having a question on gas measurements of hydrogen cooled generator. We are currently measuring:

1. Air in CO2, 2. H2 in CO2, and 3. H2

in Air with gas analyser for our small 50 MW Fuji H2-cooled generator. There has been a 'rumour' around that it is also beneficial to measure oxygen level in the generator as well for explosion prevention purpose, in addition to H2 purity measurement. Is there a value in direct measurement of O2 in generator for explosion prevention? Does anyone have any experience with this?

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